The invention relates to a method of adapting linguistic speech models in automatic speech recognition systems by means of speech recognition results obtained during operation of the systems, in which, during the adaptation, a list of N-best recognition result alternatives with N greater than 1 for a speech utterance to be recognized is evaluated.
A corresponding on-line adaptation of speech models is particularly required in dialogue systems using automatic speech recognition. Such dialogue systems provide the possibility of, for example, speech-controlled data bank inquiries. Examples are train timetable information systems, telephone information systems, airport information systems and information systems for bank clients.
The speech recognition is performed by means of stochastic models. Both acoustic models, based on HMM models (Hidden Markov Model) and linguistic speech models which represent probability values of the occurrence of speech elements of a semantic and syntactic nature are used. Notably in dialogue systems, the problem often occurs that there is not enough training material available for training the linguistic speech models used for the speech recognition before the system is taken into operation. For this reason, it is desirable to provide an on-line adaptation in dialogue systems in which the speech recognition results obtained during operation are used for further improvement of the used linguistic speech model or for adaptation to the relevant fields of application. Such an adaptation is designated as being unsupervised because only the speech recognition result found is available to the speech recognizer, rather than safe information about the actually provided speech utterance.
It is known from S. Homma et al, xe2x80x9cImproved Estimation of Supervision in Unsupervised Speaker Adaptationxe2x80x9d, ICASSP 1997, pp. 1023-1026 to use the best recognition result alternative, i.e. the one having the greatest probability, only for the on-line adaptation in the case of unsupervised on-line adaptation of linguistic speech models from a list of N-best recognition result alternatives defined for a speech utterance, when the difference between this probability and the probability of the second best recognition result alternative exceeds a given predetermined threshold value.
It is an object of the invention to improve the on-line adaptation of the linguistic speech models.
This object is solved in that a combination of a plurality of recognition result alternatives of the list is included in the adaptation.
This has the advantage that a compensation is created for those cases where the element of the list of N-best recognition result alternatives evaluated as the best recognition result alternative does not correspond to the actually provided speech utterance. This will be regularly represented by at least another recognition result alternative of the list. By combining a plurality of recognition result alternatives of the list in accordance with the invention, an error-reducing compensation is achieved in such cases, which eventually leads to an improved on-line adaptation regarding linguistic speech modeling.
Particularly when recognizing sequences of single speech elements, which are combined to a speech utterance, the invention benefits from the fact that single speech elements of the actual speech utterance may not be represented in the best recognition alternative but with great probability in at least one of the other list elements of the list of N-best recognition result alternatives. In on-line adaptation, such parts of recognition result alternatives are not ignored but taken into account with a given weight. Furthermore, in cases where speech elements are represented in the best recognition result alternative of the list of N-best recognition result alternatives, which speech elements were not part of the actual speech utterance, it is very probable that such speech elements are not represented in the other list elements. Here, too, the fact that further list elements are taken into account provides a compensation for an error which would occur when only the best list element were taken into account.
The inventive idea is realized in that, in the combination of recognition result alternatives of the list, probability values assigned to these alternatives are weighted with a given numerical value, and an adaptation weight for a recognition result alternative used for the adaptation is formed in that the weighted probability value assigned to this recognition result alternative is related to the sum of the weighted probability values assigned to the other recognition result alternatives of the list. This implementation, which can easily be realized and leads to satisfactory adaptation results, is made more concrete in that the adaptation weights are defined in accordance with the formula       ω    i    =            l      i      λ                      ∑                  j          =          1                N            ⁢              l        j        λ            
in which
xcfx89i is the adaptation weight relating to the ith element of the list of N-best recognition result alternatives, and
li is the probability value of the ith element of the list of N-best recognition result alternatives. The weight xcex can be determined heuristically for each case. When the probability values li are present in a logarithmic form, this formulation has the advantage that the involution with the weight xcex changes over to a multiplication by this weight.
The invention relates to a speech recognition system wherein a linguistic speech model used for speech recognition is adapted in accordance with any one of the methods described above.